The invention relates to video timing generation.
As shown in FIG. 1, a video image is typically generated on a television screen 10 by projecting three electron beams (one for red, green, and blue) onto a phosphor coating on the back of the screen 10. Each electron beam traverses a predetermined pattern across the screen 10, and the time varying intensity of the electron beam (i.e., the number of electrons striking the phosphor coating) modulates the intensity of light emitted by the phosphor coating.
The predetermined pattern includes horizontal scan lines 12 with the first scan line 12 having an origin at a point 16 at the upper left corner of the screen 10. Each scan line 12 has a slight downward incline from the left to the right of the screen 10. Once the electron beam reaches the far right of the screen 10 (and current scan line 12), the electron beam traverses horizontal retrace lines 14 to move the electron beam back to the far left of the screen 10.
For noninterlaced video, once the electron beam reaches a point 18 at the bottom of the screen 10, the top-to-bottom scan, or field, and the video image, or frame, are complete. Each electron beam then returns to the point 16 via a vertical retrace line (not shown). For interlaced video, a frame is formed from multiple successive scans of the electron beam from the top of the screen 10 to the bottom, i.e., multiple fields are used.
A typical video signal typically has horizontal timing signals which contain image information for controlling the three electron beams. The type of horizontal timing signal appearing in the video signal is a function of a vertical timing of the video signal. As shown in FIG. 2, the video signal may be divided into three fundamental types of horizontal timing signals: a regular horizontal timing signal 35, a serration pulse signal 36, and an equalization pulse signal 37.
Within the regular horizontal timing signal 35 an active signal 32 (in an active region 31) contains the information for one scan line 12. The time-varying magnitude of the horizontal timing signals 35-37 indicate the black-and-white formation for the scan line 12. The minimum black-and-white level (a black level) of the horizontal signals 35-37 is equivalent to the color black, and the maximum black-and-white level of the horizontal signals 35-37 is equivalent to the color white. Thus, the minimum level of the active signal 32 is equal to the black level.
A synchronization level is the minimum voltage level of the horizontal timing signals 35-37 and cannot be seen on the screen 10. Thus, during a region 33 of the regular horizontal timing signal 35 associated with one of the vertical retrace lines 14, the horizontal timing signal 35 is equal to the synchronization level. The regular horizontal timing signal 35 also has a region 29 which contains a color burst signal 34. The color burst signal 34 is a reference signal used to extract color information from the active signal 32.
The serration 36 and equalization 37 pulse signals occur during video blanking (i.e., during times when no additional illumination of the screen 10 occurs), such as during the generation of the vertical retrace line. The serration 36 and equalization 37 pulse signals can be distinguished by the length of the regions 38 and 42, respectively, at which the signals are at the synchronization level.